Foot drop is a severely disabling condition that commonly occurs as a consequence of a cerebral vascular accident or central neurological lesion in the cortex, brain stem and/or spinal cord, such as can occur with stroke, multiple sclerosis, brain cancer, traumatic brain injury, incomplete spinal cord injury, or cerebral palsy. The resulting paralysis or paresis in the ankle dorsiflexor muscles prevents a person from lifting the affected foot such that the foot is dragged and the toes stumble during walking. When the cause of the foot drop is a central neurological lesion, the ankle dorsiflexor musculature and the peripheral innervation via the common peroneal nerve are physiologically intact. However, as a consequence of the paralysis or paresis, a complex pathological sequelae develops that includes ankle extensor muscle spasticity, clonus and hyperextension, weakness in hip and knee flexor muscles, marked atrophy in the ankle dorsiflexor muscles due to disuse, compensatory gait and posture abnormalities that include hip circumduction and knee hyperextension, and pain and chronic joint problems frequently result from gait and posture abnormalities.
The foot drop disability commonly results in several impairments in gait, such as asymmetries, poor weight support in the affected leg, slow and laborious walking, impairments in posture and balance, as well as reduced endurance, rapid fatigue and high metabolic cost of walking. The foot drop condition interferes with activities of daily living such as mobility and independence and increase morbidity through increased risk of falls and hip fracture.
Until now, people suffering from foot drop have been treated primarily with ankle-foot orthoses, which are braces that clamp the ankle joint at a set angle, allowing limited or no mobility. This helps the foot drop patient when walking, since the toes no longer stumble. However, the treatment has numerous problems. The sustained pressure applied to the leg and the skin below the knee can cause pain, skin breakdown, and even peroneal nerve palsy. Use of a brace also reinforces the process of progressive atrophy of the disused dorsiflexor muscles and the onset of osteoporosis. The brace must be donned and doffed and may not be cosmetically acceptable, especially by children.
A more advanced approach for treating foot drop involves the application of electrical stimulation (ES), also known as functional electrical stimulation (FES) or functional neuromuscular stimulation (FNS) to the peroneal nerve in order to electrically activate the paralyzed or paretic muscles at appropriate times to lift the foot during walking.
Currently available, the FES systems for treating foot drop have been either totally external to the body or partially implanted with certain components implanted and other components external to the body. The totally external stimulation systems typically consist of stimulation electrodes that are placed on the skin over the area below the knee where the common peroneal nerve courses quite superficially, as well as a battery power source, a sensor (typically either a foot contact electromechanical switch worn in or under the shoe, or an accelerometer-based movement sensor) and the stimulation control circuitry. Problems with totally external stimulation systems have included excessive foot eversion produced with a single channel of stimulation, difficulties with electrode placement, variable results because of electrode movement or changes to skin conductance, pain or discomfort caused by the skin stimulation as well as the requirement for daily donning and doffing. Frequent malfunctions and breakdowns of mechanical switches and external cables are also common. Furthermore, such systems are cosmetically poor.
Historically, the Neuromuscular Assist (NMA) developed in 1968-1977 by McNeal, Perry and Waters at Rancho Los Amigos Hospital, CA was the first partially implanted peroneal nerve stimulation system. It consisted of three parts: an external power source and RF transmitter, an implanted receiver connected to a bipolar electrode wrapped around the peroneal nerve, and a heel switch worn inside the shoe that initiated stimulation during the swing phase. This team implanted 16 patients between November 1971 and January 1974 and did a two-year follow-up study. Correction of foot drop was obtained in 13 of 16 subjects. Failures were due to infection, peroneal palsy, or discontinued use. Stimulation voltages were stable after six weeks and the conduction velocity of the peroneal nerve was normal throughout the duration of the study. Dorsiflexion torque (supramaximal stimulation) remained above immediate postoperative values. However, the NMA device proved not to be practical. One of the original developers of the NMA reported the following limitations with that device:                Intact cognition and considerable patient motivation were required to wear and operate the equipment on a daily basis; and        It was difficult to obtain balanced dorsiflexion with a single channel of stimulation.        
Dr. Waters concluded in 1977 that broad acceptance of a foot drop correction system will require a totally implanted system and he also anticipated then that multi-channel peroneal stimulation will be necessary in order to obtain more balanced dorsiflexion. In the intervening time, numerous other FES-based foot drop systems have been developed but, prior to the present invention, no totally implanted foot drop correction device has been developed.
In partially implanted systems used to date, the implanted components typically include one or more stimulation electrodes, an implanted receiving antenna, and sometimes some stimulation circuitry. The external components generally include the power source, sensors, and circuitry or a computer to detect the occurrence of gait-related events in the sensed signal, as well as external antenna for communication and power transmission. Some partially implanted FES systems for treating foot drop use radio frequency telemetry communication for on-line control of the stimulation circuits by the external closed-loop control unit and sometimes also for communication between the external sensors and the external control circuitry. Problems with partially implanted FES systems for treating foot drop include: the need to don/doff the external components, crude on-off control, and the requirement that users must wear a shoe when the foot switches are used. Finally, currently available systems have been known to exhibit frequent breakdown of mechanical sensors or external connecting wires and problems with aligning the internal and external antenna for safe communication and operation.
More advanced, partially implanted systems under development are designed to detect a physiological signal that is gait-related and can be sensed by implanted nerve sensing electrodes, such as a nerve cuff electrode placed on a peripheral nerve supplying the foot sole, and such sensed nerve signal may be used as feedback signal to control a stimulation device, as was originally taught by Hoffer (1988). Strange and Hoffer (1999a, 1999b) implemented in the cat forelimb a real time FES state controller that was designed to sense natural sensory nerve signals using nerve cuff electrodes and use the sensed signals as feedback to control the timing of activation with FES of the Palmaris Longus muscle during walking on a treadmill. Thus, the principle of using sensory nerve signals as feedback to control FES of paralyzed or paretic muscles is well understood. However, until now it has been impossible to implement feedback from nerve signals in fully implanted, clinically applicable FES systems, because of the unavailability of suitable implantable circuitry to amplify and process the signals sensed from nerves and to reliably detect the needed gait-related event information from the sensed nerve signals.
However, a limitation in the ability to implement this approach until now has been the unavailability of suitable implantable circuitry to amplify and process the signals sensed from nerves and to reliably detect the gait-related events from the sensed nerve signals.
Given these problems and limitations of existing foot drop systems, there is a need for a low power, totally implantable, event-triggered, closed-loop electrical stimulation system for controlling foot drop or other disorders to restore appropriate movement to an impaired foot, leg or other body part as needed for walking, postural adjustments, or other normal activities for daily living.